Solid industrial wastes containing minor amounts of water soluble chromium, such as those residues which result from the processing of chromium ores, will bleed chromium salts when wetted by exposure to rain. Such bleeding is objectionable, as soluble chromium compounds are toxic, and constitute a serious pollutant. Some of these water soluble chromium compounds have a relatively low degree of solubility and dissolve very slowly, hence cannot be readily leached to exhaustion. They nevertheless have sufficient solubility to pollute the environment by such bleeding long after they have been discarded. The present invention relates to a method of treating such solid wastes so that no appreciable quantity of chromium will bleed therefrom.
Solid industrial wastes containing minor amounts of water soluble chromium, such as those mineral wastes resulting from the manufacture of chromic acid and chromates, present a serious disposal problem long recognized by the industry. These wastes are generally discarded by stockpiling outdoors. If they consisted only of inert insoluble residues, they would pose no problem or threat to the ecology, but unfortunately, no matter how efficient the leaching process, some soluble toxic chromium salts remain. When such stockpiles are exposed to the elements and wetted by rain, these salts are gradually leached from the residue over long periods of time to pollute the ground water. It is clear that to stockpile these wastes where they would not be wetted, would be difficult and expensive, therefore some means of minimizing or eliminating the tendency of these wastes to pollute their environment with toxic chromium compounds is badly needed.
The fact that these residues continue to bleed soluble chromium salts on wetting, even though previously subjected to efficient leaching methods, is understandable when it is considered that the residue obtained in the processing of chromium ore generally contains small amounts of calcium chromate, CaCrO.sub.4, and calcium alumino-chromate, 3CaO.sub.x.Al.sub.2 O.sub.3.CaCrO.sub.4.12H.sub.2 O, which are very slowly soluble in water; tribasic calcium chromate, Ca.sub.3 (CrO.sub.4).sub.2, which decomposes slowly in the presence of water to produce hexavalent chromium and insoluble trivalent chrominum hydroxide; and basic ferric chromate, Fe(OH)CrO.sub.4 which hydrolyzes slowly in water, releasing chromate ions. The waste residue also contains some trivalent chromium compounds, but these are insoluble in water and do not contribute to the pollution problem.
In the manufacture of the chromium compounds it has generally been the practice to mix ground chrome ore with lime and soda ash, then calcine it at high temperature in an oxidizing atmosphere whereby a large proportion of the chromium in the ore is converted to sodium chromate. This is extracted from the calcine by leaching with water. The spent ore consists mainly of a complex mixture of cement-like calcium compounds such as silicates, aluminates, and alumino-ferrites derived from the action of the lime on the gangue and other non-chromium constituents of the chrome ore during the calcining operation.
As previously mentioned, despite highly efficient leaching methods, the waste residue generally contains minor amounts of unleached, water soluble chromium compounds, and these dissolve in water at widely different rates. These residual, slowly soluble chromium compounds generally are present to the extent of from 0.7 to 2.0 percent Cr on a weight basis, but can vary widely from about 0.5 to 5 percent or more. Such residue when wetted will generally bleed chromium salts to an extent such that the bleed will contain about 100 to 500 or more parts per million (ppm) as Cr. This amount is much above that normally considered acceptable. Generally, it is desirable to have the Cr content of the bleed amount to less than 25 ppm and if possible, less than 10 ppm. Of course, the ideal situation would be to have a zero chromium content or substantially zero (i.e. a fraction of 1.0 ppm).